The present invention relates to a improved method of HgCdTe liquid phase epitaxy (LPE).
"HgCdTe" is used generically to refer to the family of Hg.sub.1-x Cd.sub.x Te alloys. HgCdTe is an exceedingly important material for infrared detection and imaging applications, since its band gap can be arbitrarily selected to be as small as desired (by varying the alloy composition). However, HgCdTe is a very difficult material to fabricate, for two major reasons: first, the partial pressure of mercury at temperatures of interest for HgCdTe crystal growth or LPE is so high that out-gassing of mercury, and consequent loss of stoichiometry, is a great hazard. Second, HgCdTe permits freezing out of numerous successively different fractions having different alloy compositions at only slightly different temperatures (incongruent solidification). Since any variation in composition alters the desired electronic properties, extremely precise control of the melt temperature during liquid phase epitaxy is necessary. In particular, the degree of supercooling used must be precisely controlled, since excessive supercooling may induce solidification of undesired alloys. A meltback step, prior to deposition of the desired alloy, is often required to remove the seeds of unwanted alloys previously deposited. This step also imposes stringent temperature - control requirements on any practical LPE process. The present invention is specifically aimed at solving these latter problems.
In the prior art, attempts have been made to determine the liquidus temperature of HgCdTe alloys by sealing a small sample of the alloy in a fused-silica ampoule, and then performing differential thermal analysis (DTA) with respect to a neutral body. The liquidus temperatures thus measured were used to attempt to determine the growth temperature for HgCdTe LPE in a subsequent production run. However, a crucial disadvantage of this indirect method is that the solution in the reactor may not possess the same liquidus temperature as that of the specimen sealed in the small ampoule, because as noted above, composition changes due to mercury vaporization are a constant source of uncertainty in HgCdTe. Thus, this indirect technique does not actually measure the true liquidus temperature of the melt, and therefore does not provide a sufficiently reliable technique for precise production control.
Applicants also note the following publications on HgCdTe properties and LPE, which are hereby incorporated by reference:
Long and Schmit, "Mercury-Cadmium Telluride and Closely Related Alloys," in 5 Semiconductors and Semimetals 175 (ed. R. Willardson and A. Beer, 1970); PA1 Wang et al., "Liquid Phase Growth of HgCdTe Epitaxial Layers," 127 J. Electrochem. Soc.: Solid- State Science 175 (1980); PA1 Mroczkowski and Vydyanath, "Liquid Phase Epitaxial Growth of (Hg.sub.1-x Cd.sub.x) Te from Tellurium-Rich Solutions Using a Closed Tube Tipping Technique," 128 J. Electrochem. Soc.: Solid-State Science 655 (1981); PA1 Harman, "Liquidus Isotherms, Solidus Lines and LPE Growth, in the Te-Rich Corner of the Hg-Cd-Te system," 9 J. Electronic Materials 945 (1980), and PA1 Bowers et al., "Comparison at Hg.sub.0.6 Cd.sub.0.4 Te LPE Layer Growth from Te-, Hg-, and HgTe-Rich Solutions," 27 IEEE Trans'ns on Electron Devices 52 (Jan. 1980). PA1 providing a reactor to contain a melt of said desired material; PA1 providing a neutral body; PA1 furnace heating said neutral body and said reactor in mutual proximity; PA1 differentially sensing the temperature difference between said neutral body and said melt; PA1 identifying the liquidus temperature of said melt at a sharp change in the temperature difference detected by said differential sensing step; and PA1 cooling said melt at a desired rate to a desired temperature below said liquidus temperature, to effect a desired crystal growth sequence.
Thus, it is an object of the present invention to provide a method for continuously and precisely monitoring the temperature of a high-temperature liquid mixture while in a furnace.
It is a further object of the present invention to provide a method for continually and precisely monitoring the temperature of an HgCdTe melt while in a furnace.
It is a further object of the present invention to provide a method for continually and precisely monitoring the temperature of an HgCdTe melt during liquid phase epitaxy.
It is a further object of the present invention to provide an improved method for liquid phase epitaxy of electronic materials having incongruent solidification.
It is a further object of the present invention to provide a method for reliably and uniformly controlled liquid phase epitaxy of HgCdTe.